Historically, electron devices in the first several decades of the 20th century required vacuum tight housings to support the propagation of an electron flux therein. These housings were hermetic structures of various materials and took on a variety of forms requiring a corresponding variety of equipment to fabricate. A very significant part of the cost of any such device was associated with the hermetic-sealed housing. During the last several decades of the century, solid state electron devices evolved for which there was no such vacuum requirement. There remain classes of electron devices which require formation and control of an electron flux in the vacuum environment for which the vacuum tight housing remains a major economic and operational consideration. Typical of these devices are x-ray sources, and image detection devices. Requirements for large scale production efficiencies and increased device complexities motivate an evolutionary approach to the design and fabrication of the package for micro-electronic devices. Generally desirable specifications for the housing would recognize the need to miniaturize the entire package; to assure an inherently low cost for materials and fabrication; to reduce the part count per device to obtain high yield in the manufacturing process; to employ conventionally available capital equipment; to obtain housings which can be characterized by a standard format; and in appropriate devices, to obtain a satisfactory isolation of any applied high potentials in the miniaturized device scale.
Consider the cooperative benefits of the above enumerated desiderata: a conventional standard form factor may be associated with existing classes of sockets and with existing equipment for surface mounting such devices on printed circuit boards. Unusually added complexities in the form of increased numbers of signal leads can be accommodated in such standard form factors, e.g., plastic leaded chip carrier (PLCC) type socketing hardware. In classic vacuum tubes 8, 12 and 16 leads inserted into the vacuum housing represented a significant level of complexity for the purposes of the device and for its fabrication. Contemporary PLCC sockets accommodate many leads. As many as 128 leads is a common requirement for modern integrated circuits. Such a number of signal and control leads is not unusual for an image detector array, by way of example.
Certain genera of fabrication processes practiced for producing packages for semiconductor devices are employed herein for the novel purpose of achieving vacuum tight housings for microelectronic devices. In the present work, reference will be repeatedly made to the example of a class of image detection devices employing electron bombarded active pixel arrays.
“Tape casting” is a well known form of fabrication of ceramic objects in the area of semiconductor packages. The term refers to a series of steps and resulting structures, wherein a ceramic slurry is created from selected ceramic precursors and additives for the particular purpose which are mixed on a flat work surface to produce a planar layer for an eventual multi-layered structure. A doctor blade or like instrument is then drawn over the slurry at a selected rate to obtain a uniform material thickness for that component layer. An aperture of specified dimensions is then removed from the interior of the constituent layer. The slurry is then allowed to dry in air and the result is known as a “green tape”. Depending upon the additives, the green tape is flexible and sufficiently robust to tolerate reasonable handling. The tape is cut to size and a stack of green tape constituent layers is assembled to define a package for housing a semiconductor device. In the context of conventional semiconductor packaging, electrical leads may be printed with refractory metal-based inks deposited on surfaces of one or more component layers to provide electrical communication paths from the interior of the package to the exterior thereof. The stacked green tape assembly is then sintered at selected temperatures of the order of 1500° C. to produce a monolithic structure from the multi-layered composite into which the semiconductor chip is mounted, wire bonded to pads connected to the printed leads and the housing is then closed. Tape casting is a well known process for assembling ceramic packages for semiconductor devices. Typical references are “Multilayer Ceramics: Design Guidelines” (Kyocera, CAT/2T9203THA/1242E, 1992) and “Design Guide” (Coors Electronic Package Company, 1998).
In U.S. Pat. No. 5,581,151, a vacuum electronic image detector is known in which a cylindrical housing is formed from a layered ceramic structure, cofired to form a unitary ceramic structure. In this known structure, all control and signal leads (other than the photocathode) are lead through vias to pins downwardly projecting from the base of the housing. The plurality of layers forming this cylindrical known structure define an internal cylindrical cavity comprising a stepped arrangement of sequentially greater (lesser) diameter to support, or form component parts of the structure. Additionally, this prior art achieves a vacuum seal incorporating a flange brazed to the package body to adhere to an indium metal seal to a window, an arrangement that adds cost and processing complexity.
It is known in prior art to employ cold, crushed Indium for vacuum sealing. A representative reference is C. C. Lim, Review of Scientific Instruments, vol. 57, pp. 108-114 (1986).